Polyurethane elastomers are conventionally prepared by reacting a prepolymer with a chain extender. Typically, the prepolymer comprises the reaction product of a polyol and a diisocyanate monomer with excess molar amounts of the diisocyanate monomer. As such, the isocyanate groups of the diisocyanate “cap” the hydroxy groups of the polyol resulting in an isocyanate terminated prepolymer. The most commonly used prepolymer mixtures utilize diphenylmethane diisocyanate (“MDI”) and toluene diisocyanate (“TDI”) as the diisocyanate monomer.
The resulting prepolymer is then cured with a chain extender to form the final polyurethane product. The chain extender links multiple diisocyanate monomers to form the resultant polyurethane. Typical chain extenders include aromatic amines such as methylene bis orthochloroaniline (“MOCA”), methylene bis diethylaniline (“MDEA”), methylene bis chlorodiethylaniline (“MCDEA”), and hydroquinone-bis-hydroxyethyl ether (“HQEE”), and 4,4′-methylene-bis(2-chloroaniline) (“MBCA”); and diols, e.g., ethylene glycol (“EG”), diethylene glycol (“DEG”), triethylene glycol (“TEG”), propylene glycol (“PG”), dipropylene glycol (“DPG”), and 1,4-butanediol (“BDO”).
Another class of chain extenders is metal salt coordination complexes of methylenedianiline (“MDA”). In these complexes, the MDA is blocked by a reaction with a metal salt to form a coordination complex. Typically, it is necessary to de-block the MDA from the coordination complex before the MDA can effectively chain extend the respective prepolymer. This may be done by applying heat, for example, to a mold that contains the prepolymer and the chain extender.
In some applications, the reaction of the prepolymer and the chain extender takes place too slowly, e.g., the chain extender does not cure the prepolymer and the polyurethane is not formed, e.g., polymerized, quickly enough. As a result of the slow cure rates, cure temperatures must be increased and/or cycle times must be lengthened, both of which result in decreased productivity. In other applications, the de-blocking of the MDA coordination complex occurs more rapidly at the surface of the heated mold. Thus, the uniformity of the structure of the resultant polyurethane elastomer may not be consistent throughout. As such, a hard skin may first form on the outer surfaces and, as the cure proceeds, the skin may rupture, resulting in an undesirable cracked surface.
Some cure accelerators are known such as glycerol and urea. Such cure accelerators, however, do not work well with all prepolymer/chain extender combinations. In addition, the performance demonstrated by these cure accelerators leaves much room for improvement.
Thus, the need exists for accelerating the reaction between prepolymers and chain extenders in order to increase productivity and to produce polyurethane elastomers, in particular, high performance polyurethane elastomers, that ideally have a substantially uniform consistency.